Geothermal wells are drilled in subterranean formations with high temperatures. Heat is extracted from the formation through the well for use in surface electric power generation systems. Typical locations for suitable formations include areas close to volcanoes and tectonic plate interfaces. These target formations are usually located at relatively shallow subterranean depths (3000 to 8000 ft). In situ temperatures for these and other geothermal formations can range from 350-600° F.
Reservoirs targeted for conventional geothermal operations and Engineered Geothermal Systems (“EGS”) operations are commonly non-permeable high temperature reservoirs containing naturally occurring fractures. Conventional geothermal wells are drilled in hot formations containing water. EGS wells are used to extract heat from high-temperature formations containing no liquids or relatively low amounts of liquid. In EGS formations, water can be injected into one well, circulated through fractures in the geothermal reservoir, and then produced out of a second well as hot geothermal fluid. Hot geothermal fluid or steam can be used to generate electricity. Fractures in EGS formations are not typically open to fluid flow and must be opened through the injection of water and hydro-shear dilation of the formation to create a permanent flow path for water and heat exchange.
Geothermal formations can contain an extensive network of naturally existing fractures. The width of open fractures can vary from less than 1/16 inch to greater than 2 inches. The fractures pose problems during drilling and completion of conventional geothermal wells and EGS wells. The fractures provide alternate flow paths for drilling fluid which is used to control the well, cool the drill bit and carry drill cuttings from the well. Open fractures can imbibe drilling fluid thereby reducing the rate of penetration and impeding progress of the drilling operation. Closed fractures can open during drilling due to hydrostatic pressure of the drilling fluid. The loss of drilling fluid from the wellbore into an open fracture, termed lost circulation, reduces drilling effectiveness and hinders subsequent well completion operations.
In related drilling and production operations, drilling fluids primarily composed of bentonite clay and barite can be used to increase the viscosity and density of the drilling fluid. During drilling operations, lost circulation can occur and large volumes of drilling fluid can flow into a fracture in a potentially productive interval of the well. At high temperatures, solids such as, bentonite and barite can form a non-removable solid mass that impedes or totally blocks productive flow from zones in the well. These solids can also damage the formation.
Therefore, there is a need in the field of art for non-damaging drilling fluids and weighting materials. There is also a need in the field of art for improved systems, methods and compositions for sealing fractures in subterranean formations to prevent lost circulation.